Synthesis of N-methylglucamine functionalized calix[4]arene based magnetic sporopollenin for the removal of boron from aqueous environment

Desalination 310 (2013) 67–74

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Synthesis of N-methylglucamine functionalized calix[4]arene based magnetic sporopollenin for the removal of boron from aqueous environment Muhammad Afzal Kamboh, Mustafa Yilmaz ⁎ Department of Chemistry, Selcuk University, Konya-42075, Turkey

H I G H L I G H T S ► Synthesis of new N-methylglucamine functionalized calix[4]arene based magnetic sporopollenin ► Application of calix[4]arene-based magnetic material for boron sorption ► The boron sorption is highly dependent on salinity as well as pH of boron solution.

a r t i c l e

i n f o

Article history: Received 12 August 2012 Received in revised form 25 October 2012 Accepted 26 October 2012 Available online 21 November 2012 Keywords: Calixarene Sporopollenin Nanoparticles Sorption Boron

a b s t r a c t The present study deals with the synthesis of new N-methylglucamine functionalized calix[4]arene based magnetic sporopollenin (4) and its application for the removal of boron from aqueous environment. The newly prepared calix[4]arene immobilized material 4 is characterized by SEM, EDS and FT-IR spectroscopy. The sorption efficiency of newly calix[4]arene-based magnetic material 4 for boron from aqueous media was evaluated through the solid–liquid sorption experiments. During the sorption process, various kinds of interactions such as electrostatic repulsion, deprotonation of the hydroxyl groups of sorbent 4, and dissociation of H3BO3 into anions/cations were monitored. The sorption of boron on material 4 is highly pH dependent and significant percent sorption (84%) was achieved at pH = 7.5. The sorption behavior was analyzed by Langmuir and Freundlich isotherms. The values of correlation coefficients (R 2) showed that the Langmuir isotherm model found to be best fit. Experimental data revealed that material 4 is an efficient sorbent which can be effectively used for the boron decontamination of aqueous media. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nowadays by virtue of hasty progress in the meadow of science and technology human being has reached number of milestones in the field of industrialization. But at the same time this swift proliferation of industrial units by virtue of the industrial wastes is also playing a crucial role in extensive global contamination [1–3]. Especially water contamination as a result of industrial wastes is one of the major problems of modern era [4]. Industrial sector produces a huge quantity of wastewater generally contains variety of pollutants such as, toxic metals, dyes, pesticides and boron [5–7]. Among the industrial sectors particularly glass and detergent industries are recognized as the major sources of boron contamination [8]. However, boron contamination as a result of natural or anthropogenic sources is of prime importance due to the high risks concerning from the boron toxicity as well as its detrimental effects on the reproducibility of living organisms [9,10]. No doubt at low doses boron is not only an essential micro nutrient for plants and humans, but may also be potentially toxic at elevated concentration

⁎ Corresponding author. Tel.: +90 332 2233873; fax: +90 332 2410106. E-mail address: [email protected] (M. Yilmaz). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.10.034

[11]. In plants generally boron exhibits many biological functions such as carbohydrate metabolism, sugar translocation, pollen germination, hormone action and nucleic acid synthesis. On the other hand elevated concentration of boron in plants retards the photosynthesis which shows adverse effect on the growth as well as productivity of plants [12–14]. Beside this, boron is considered as a necessitate component of human's daily diet, because it actively participates in a broad variety of metabolic processes such as, metabolism, utilization of calcium and vitamin D in humans, lignifications, membrane transport and enzyme interactions. While, excessive administration of boron may cause nausea, vomiting, diarrhea and atrophy in the humans [15,16]. Consequently, taking into consideration the detrimental effects World Health Organization (WHO) declared the safe concentration levels of boron for plants and humans. According to the WHO requirements, the approved concentration of boron for drinking water is 0.3 mg/L, while for irrigation purpose it is variable i.e., 0.3 mg/L, 1–2 mg/L and 2–4 mg/L for sensitive, semi-sensitive and tolerant plants respectively. But a very high concentration of boron (i.e., 1–63 mg/L) in aquifers has been reported [9]. Therefore, by virtue of well known toxicity the precise determination and removal of boron from aqueous system is of significant importance [10]. Up till now many researchers have done considerable efforts to decontaminate the boron contaminated effluents through the different

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2. Experimental

technologies [2,11,16–18]. Comparatively, sorption technique due to its simplicity, rapidity and relatively low cost has gained a lot of popularity in the recent years. Since, boron sorption is based on the complexation between the surface of sorbent and boron, so due to the low selectivity conventional sorbents such as oxides, clays, fly ash and activated carbons cannot be effectively use [11,16]. Therefore, utilization of new highly selective boron chelating sorbents which can be effectively used in aqueous media has currently become a focus of intense research. It is well known that, N-methylglucamine grafted materials are considered as most promising boron sorbents [19]. Consequently, in this study we have reported the preparation of N-methylglucamine functionalized calix[4]arene based magnetic sporopollenin. The chemical immobilization of calix[n]arene framework onto the magnetic sporopollenin makes such macrocycle a versatile material that not only helps in the remediation of polluted sites but also it provides chemical, physical as well as thermal stability [20–22]. The main objective of the present study is to evaluate the sorption efficiency of newly synthesized calix[4]arene-based magnetic material i.e., 4 towards the boron from aqueous media.

2.1. Reagents TLC analyses were carried out on DC Alufolien Kieselgel 60 PF254, Merck Darmstadt, Germany. Lycopodium clavatum with a particle size of 25 μm was purchased from Fluka Chemicals. All chemicals used were of analytical grade and purchased from Merck or Aldrich (Aldrich; Steinheim, Germany) and used without further purification. All commercial grade solvents were distilled and then stored over molecular sieves (Aldrich, 4 Å, 8–12 mesh). The pH of the solution was adjusted by mixing appropriate amount of HCl and/or NaOH (0.1 N). Deionized water that had been passed through a Milli-Q system was used for the preparation of solutions. 2.2. Apparatus Elemental analysis was performed using a Leco CHNS-932 analyzer. Melting points were determined on a Gallenkamp apparatus (UK) in a

OH OH OH HO

OH OH OH HO

OH OH OH HO N

N (3)

HO (2)

OH

OH

HO

OH

HO

HO

(1)

OH OH

HO

Si(OC2H5)4 1%NaF (I) (II)

iv

(3)

Sporopollenin

(4)

Fig. 1. The synthetic routs for the preparation of N-methylglucamine functionalized calix[4]arene-based magnetic sporopollenin (4). Reaction conditions: (i) AlCl3, phenol, toluene; (ii) N-methylglucamine, formaldehyde, CH3COOH, THF; and (iii) EPPTMS-MS, K2CO3, CH3CN.

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sealed capillary glass tube. 1H NMR spectra were recorded on (Varian 400 MHz, UK) spectrometer. IR spectra were recorded with a (FTIR, PerkinElmer 1605, USA) spectrometer as KBr pellets. UV–VIS spectra were obtained with a (UV-160A, Shimadzu, Japan) UV–visible spectrophotometer. An Orion 410A+pH meter was used for the pH measurements. SEM analysis was performed on Zeiss EVO LS 10 scanning electron microscope. 2.3. Synthesis p-tert-Butylcalix[4]arene (1), calix[4]arene (2) and N-methylglucamine derivative of calix[4]arene (3) as illustrated in Fig. 1 were synthesized according to the previously published methods [23–25]. 2.3.1. Preparation of magnetic sporopollenin (MS) Magnetic sporopollenin (I) as illustrated in Fig. 1 was prepared as follows: 13.32 g FeCl3 6H2O, 19.88 g FeCl2 4H2O, 5 mL 5 M HCl, 40 mL milliQ water and 5 mL ethanol were mixed in a 100 mL flask followed by heating to 40 °C until complete dissolution of the salts. Then 1 g sporopollenin was redispersed in 30 mL of this solution and stirred for 2 h at room temperature. The sporopollenin suspension was filtered and quickly washed with milliQ water on the filter and then an immediately transfer into 1 M ammonia solution in milliQ water. After 2 h stirring at room temperature, the “magnetic sporopollenin” (with cores of magnetite nanoparticles) were collected by magnet and washed thoroughly with milliQ water and dried under vacuum [26]. The IR spectral data of (1) (KBr disk): 3335 cm−1 (OH), 1742 cm−1 (C_O), 1517 cm−1 (C_C), 1442 cm−1 (C\C), 1148 and 1111 cm−1 (C\O) and 573 cm−1 (Fe\O). 2.3.2. Preparation of [3-(2,3-epoxypropoxy) propyl] trimethoxysilane (EPPTMS)-grafted magnetic sporopollenin (EPPTMS-MS), II EPPTMS grafted magnetic sporopollenin (II) as illustrated in Fig. 1 was prepared by the reaction between EPPTMS and the hydroxyl groups on the surface of magnetite. Typically, 1.0 g of magnetic sporopollenin was suspended in 50 mL of distilled water. A mixture of 5 mL of EPPTMS, 10 mL of methanol and 4 mL of 1% NaF aqueous solution was stirred for 5 min. After that 0.5 mL of tetraethyl orthosilicate was added drop wise and resulting solution was stirred for 60 h at room temperature. The forming product was collected through

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magnet and thoroughly washed with ethanol and deionized water until reaching neutral pH and dried under vacuum. The IR spectral data of the EPPTMS-MS carriers (KBr disk): 3340 cm −1 (OH), 1514 cm −1 (C_C), 1438 cm −1 (C\C), 1340, 1251, 1089 and 1031 cm −1 (C\O) and (Si\O) groups respectively.

2.3.3. Immobilization of N-methylglucamine functionalized calix[4]arene (3) onto the (EPPTMS-MS) II The immobilization of N-methylglucamine functionalized calix[4] arene (3) onto the (EPPTMS-MS) II, as illustrated in Fig. 1 was carried out as follow. A mixture of the compound 3 (0.3 g, 0.358 mmol) and potassium carbonate (0.5 g, 3.6 mmol) in acetonitrile (30 mL) was stirred at 80 °C for 30 min. Then 1 g of freshly prepared (EPPTMS-MS) II was added to the reaction mixture followed by the refluxing for 80 h. The immobilization was followed by the disappearance of calixarene from the reaction solution by UV spectrophotometry (λmax: 282 nm) and FT-IR spectroscopy. The resulting N-methylglucamine functionalized calix[4]arene based magnetic sporopollenin was isolated through the magnetic separation and washed with surplus DMF in order to remove excess compound 3, then washed with water and dried under vacuum.

2.4. Sorption procedure for boron 2.4.1. Batch method (solid–liquid sorption) The batch sorption study was carried out to evaluate the boron removal efficiency of newly synthesized sorbent i.e., 4 as illustrated in Fig. 1. Different parameters such as sorbent dosage, pH, effect of electrolyte and contact time were optimized to achieve better sorption results. The experiments were conducted in 25 mL Erlenmeyer flasks with glass cap which contain particular amount of the sorbent as well as particular concentration of sorbate solution i.e., 5 ppm aqueous solution of H3BO3 acid. To obtain sorption equilibrium, the Erlenmeyer flasks were stirred on a horizontal shaker operating at a constant speed (170 rpm) at 25 °C for 1 h. Then the sorbent was removed by magnetic separation. The boron concentration before the treatment as well the residual concentration of boron in aqueous phase after the treatment was analyzed through UV–vis spectrophotometer at 411 nm by using azomethine-H as a colorimetric reagent according to the modified azomethine-H method as described previously [27].

Fig. 2. FTIR spectra of magnetic sporopollenin (a), EPPTMS-MS (b) and calix[4]arene based magnetic sporopollenin (c).

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The % sorption of boron was calculated through the Eq. (1) as follows: % Sorption ¼

C i −C f  100 Ci

ð1Þ

where Ci (mol L −1) is initial concentration of boron before the sorption and Cf (mol L −1) is the final concentration boron after the sorption. 3. Results and discussion 3.1. Characterization

micrographs were obtained to observe the surface morphology of pure sporopollenin, magnetic sporopollenin (I) and calix[4]arene-based magnetic material 4. The SEM images as shown in (Fig. 3a, b and c) are very demonstrative. Smooth morphology of the sporopollenin network with open and uniform pore structure can be seen in Fig. 3a. SEM image of the magnetic sporopollenin (Fig. 3b) on high magnification reveals that the magnetite nanoparticles are predominantly localized at inside of the open pores of sporopollenin. Fig. 3c demonstrate the SEM image of calix[4]arene-based magnetic material 4. Following the immobilization of EPPTMS-MS with 3, as expected the surface cavity of sporopollenin treated support was filled with foreign material which is presumably N-methylglucamine functionalized calix[4]arene aggregate. The filling of sporopollenin surface cavity as well as presence of attached particles onto the surface of calix[4]arene-based magnetic material 4 confirms the immobilization [28].

3.1.1. FT-IR spectra The synthesis of (EPPTMS-MS) II as well as immobilization of N-methylglucamine functionalized calix[4]arene (3) onto the modified magnetic sporopollenin (EPPTMS-MS) II, was confirmed by FT-IR spectral analysis Fig. 2a, b and c. Magnetic sporopollenin following the modification process with EPPTMS as described in Section 2.3.2 shows some additional bands at around 1340, 1251, 1198, 1089, 1031and 902 cm−1 for C\O and Si\O groups respectively, Fig. 2b. Both EPPTMS grafted magnetic sporopollenin and N-methylglucamine functionalized calix[4]arene based magnetic sporopollenin have very similar absorption patterns but FT-IR spectral analysis shows clearly different bands at around 1656, 1514, 1375, 1251 1031 and 902 cm−1. The appearance of bands at 1656 and 1375 cm−1 as shown in Fig. 2c corresponds to N\H bending and the C\N stretching respectively. While on the other hand disappearance of characteristic peaks of epoxy group vibration from Fig. 2c at 1340, 1251, 1198, 1031 and 902 cm−1 is a qualitative evidence for the formation of material 4. Consequently, the disappearance as well appearance of some respective bands for C\O, N\H and C\N groups respectively, confirms the immobilization of N-methylglucamine functionalized calix[4]arene (3) onto the modified magnetic sporopollenin (EPPTMS-MS) i.e., II. In summary, the comparison of the two FT-IR-spectra gives reasons to believe that the epoxy groups of (EPPTMS-MS) i.e., II reacts with the phenolic carboxy groups of 3 to build a calix[4]arene-based magnetic material 4.

3.1.3. Energy dispersive spectroscopy (EDS) In ordered to investigate the purity and elemental composition of magnetic sporopollenin II and immobilized material 4, the scanning electron microscopy (SEM) was coupled with energy dispersive spectroscopy (EDS). EDS sampling was done on two different pollen grains i.e., magnetic sporopollenin II and calix[4]arene-based magnetic material 4. Fig. 4a and b presents the SEM image and EDS spectra with the elemental composition for magnetic sporopollenin II and immobilized material 4 respectively. As we know that natural ragweed pollen grains are mainly composed of hydrogen, carbon and oxygen with no iron [26]. While, the EDS results (Fig. 4a) show 4.83% of the iron in II. On the other hand EPPTMS modified magnetic sporopollenin contains no nitrogen, but after the immobilization of 3, the EDS results of material 4 (Fig. 4b) show 4.11% of the nitrogen in material 4. Consequently, by virtue of the presence of iron as well as nitrogen in materials II and 4 respectively, Fig. 4a and b shows quantitative evidences for the successful coating of magnetite onto the short ragweed pollen grains as well as the immobilization of 3 onto the EPPTMS modified magnetic sporopollenin.

3.1.2. Scanning electron microscope (SEM) Since scanning electron microscopy (SEM) is known as one of the most widely used surface diagnostic tools for that reason SEM

3.2.1. Effect of sorbent dosage To analyze the effect of sorbent dosage of material 4, on the % sorption of boron, the experiments were carried out by varying the

3.2. Sorption studies

Fig. 3. SEM photographs: pure sporopollenin (a), magnetic sporopollenin (b) and calix[4]arene based magnetic sporopollenin (c).

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Fig. 4. SEM micrograph along EDS spectra of magnetic sporopollenin II (a) and calix[4]arene based magnetic sporopollenin (b) at high magnification (10 μm).

25

50

pH study

b)

Dosage study

100 90 80 70 60 50 40 30 20 10 0

% Sorption

% Sorption

a)

100 90 80 70 60 50 40 30 20 10 0

75

5.5

6.5

Dosage (mg)

d)

Role of electrolyt

100 90 80 70 60 50 40 30 20 10 0

8.5

Effect of contact time 100

% Sorption

80

% Sorption

c)

7.5

pH

60 40 20 0

0

0.2

0.4

Conc: of NaCl (M)

0.6

0

15

30

45

60

75

90

Time (minute)

Fig. 5. Dosage effect (a), pH effect (b), electrolytic effect (c), and contact time (d) on the percent sorption of boron.

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groups (OH−). Consequently, the highest sorption value (84%) was obtained at pH 7.5. On the contrary, lower sorption above the pH 7.5 may be due to the abundance of (OH−) ions, that may had competition with B(OH)−4 for the sorption sites and thus sorption of B(OH)−4 anion is not favorable [19,30].

Table 1 Langmuir and Freundlich constants for boron sorbed onto material 4. Langmuir

Freundlich

qm (mmol g−1)

kL (L mmol−1)

R2

n

kF

R2

1.140

0.195

0.998

2.361

0.423

0.981

3.2.3. Influence of the NaCl concentration on the sorption of boron The influence of ionic strength was initially tested at different salinities i.e., the concentration of NaCl was varied from 0 to 0.6 mol L −1. Fig. 5c clearly demonstrates that boron removal efficiency improved remarkably by increasing the ionic strength. This may be due to the fact that, there will be more borate ion at higher salinity than that at lower salinity. Therefore, theoretically it is expected that boron removal will be better at higher salinity [29].

Table 2 Comparison of the boron sorption capacities of the various sorbents. Name of sorbent

Sorption capcities (mmol g−1)

Reference

Neutralized red mud Mg–Fe hydrotalcite Siral 5 Dowex 2 × 8 exchange resin Glucamine-modified MCM-41 Polyol-grafted SBA-15 Chelating resin Functionalized cellulose Functionalized polymer Material 4

0.55 0.33 0.10 1.48 0.80 0.63 1.50 1.10 2.18 1.14

[9] [31] [32] [33] [30] [34] [35] [36] [37] In current study

3.2.4. Effect of contact time The sorption of boron as a function of shaking time was studied onto material 4 at 25 °C, (sorbent dosage 50 mg and sorbate concentration 5 ppm) and the results are shown in Fig. 5d. It has been found that % sorption increases with the increase in shaking time and equilibrium was established within 1 h and there was only a minor increase in % sorption beyond the 1 h. Thus, rest of experimental work was carried out at 1 h shaking time.

amounts of sorbent from 25 to 75 mg at the fixed concentration (5 ppm) of 10 mL boric acid solution. Fig. 5a obviously demonstrates that the % sorption of boron increased with increasing the dosage of sorbent, this may be due to the availability of more sorbent surface for the boron to be sorbed. The dosage study Fig. 5a revealed that boron removal efficiency increases significantly up to the optimum dosage i.e., 50 mg beyond which the boron removal efficiency remain almost constant. Thus, all the experiments were carried out with a fixed amount of sorbent i.e., 50 mg.

3.2.5. Sorption isotherm Sorption isotherm is a functional expression that correlates the amount of solute sorbed per unit amount of the sorbent and the concentration of sorbate in bulk solution at a given temperature under equilibrium conditions. By applying the sorption models some very useful information such as sorption capacity, structure of the sorbed layer and the interaction between sorbate and sorbent can be predicted. Herein, equilibrium studies were performed and most important isotherms such as Langmuir as well as Freundlich models at optimized parameters were used to evaluate the sorption capacity of material 4. The Langmuir isotherm model assumes monolayer coverage of sorbate on a homogeneous surface of sorbent; whereas Freundlich isotherm model suggest the heterogeneity of the sorbent surface and multilayer formation [3]. The Langmuir and Freundlich isotherms are tested in the following form Eqs. (2) and (3);

3.2.2. pH effect on the sorption of boron Since, the boron sorption is based on the complexation between the surface of sorbent and boron, following the deprotonation of the hydroxyl groups of sorbent as well as dissociation of H3BO3 acid into anions/cations [16]. Therefore, pH plays an imperative role during the sorption process because, it affects on both degree of ionization of H3BO3 acid and the dissociation of functional groups on the active sites of the sorbent. In Fig. 5b, the variations of pH were plotted against the percent sorption. The highest sorption value (84%) was obtained at pH 7.5, while above pH 7.5 the sorption of boron decreased. The effect of pH during the boron sorption observed in this study may be explained on the basis of dissociation of H3BO3 acid as well as deprotonation of hydroxyl groups of N-methylglucamine unit in material 4 as briefly explained in sorption mechanism section i.e., 3.4. The H3BO3 acid on dissociation in aqueous media generates the primary species i.e., B(OH)3 as well as B(OH)−4 at (pH b 6) and (pH > 6), respectively [29]. Since, boron complexation takes place between the borate ion B(OH)−4 and hydroxyl

O B HO

O

1 log C e n

O H

OH

OH

H O

HO B O

HO

OH

OH B

+H O

ð3Þ

where qe is the equilibrium sorption capacity, Ce represents the solute concentration at equilibrium, kL (L mmol −1) is a binding constant, kF

B -H2O HO O

H OH

ð2Þ

OH

B

OH

1 1 1 ¼ þ qe kL qm C e qm

Freundlich model : log qe ¼ log kF þ

H

OH

HO

Langmuir model :

O

H O

Fig. 6. Boric acid complexation with vicinal \OH groups as well as sorption of boron by calix[4]arene based magnetic sporopollenin (4).

M.A. Kamboh, M. Yilmaz / Desalination 310 (2013) 67–74

and n are the Freundlich constants and qm (mmol g−1) is the maximal sorption capacity [19]. Table 1 shows the calculated values of the Langmuir and Freundlich equation parameters. The comparison of correlation coefficients (R2) indicates that sorption of boron onto material 4 can be better demonstrate by the Langmuir isotherm compared to the Freundlich isotherm. The applicability of the Langmuir sorption model suggests the monolayer coverage of boron on the surface of material 4. Moreover, sorption isotherms for boron suggest chemisorptions. Thus, there is a complexation between boric acid/borate and the sorbent. So chemisorptions may be preferred to physisorptions in boron removal or recovery due to its higher correlation coefficients.

3.3. Comparison of sorption capacities of material 4 with other sorbents A comparison of boron sorption capacity of material 4 with other sorbents reported in the literature [9,31–37] is given in Table 2. The data show that the sorption capacity of material 4 is relatively high when compared with the common sorbents and also shows approximately the average capacity of the boron-selective sorbents.

3.4. Sorption mechanism Boron sorption mechanism for the N-methylglucamine grafted sorbent consists of two important features: firstly, it preferably takes place with reactive functional group i.e., hydroxide group. Secondly, this reaction preferably takes place in the presences of proton able nitrogen as described in Fig. 6. Therefore, it is necessary to have at least two active centers and proton able nitrogen in the sorbent for each molecule of borate ion. According to the Simonnot et al. after dissociation of H3BO3 acid, borate ion is complexed by two sorbitol groups and a proton is retained by a tertiary amine site, which behaves as a weakly basic anion exchanger [37,38] as described below. Boric acid dissociation:

H3BO3 + H2O

+ B(OH)4 + H

Boron complexation: B(OH)4

−CH−O

+ 2 − CHOH − CHOH− = 4H2O

+

O−CH− B

−CH−O

O−CH−

þ

Amine protonation : ―CH2 ―NðCH3 Þ―CH2 ― þ H þ ¼ ―CH2 ―N HðCH3 Þ―CH2 ―:

4. Conclusion In this study a new calix[4]arene-based magnetic material 4 was synthesized and characterized by SEM, EDS and FT-IR spectroscopy. Material 4 has been used successfully as a sorbent for the removal of boron. All the results related to the removal of boron from the aqueous environment through material 4 reveal it as an effective sorbent and it was found that boron sorption is highly dependent on salinity, pH of boron solution and presence of functional groups attached to the sorbent. On the basis of these results it can be concluded that the calix[4]arene based magnetic sporopollenin (4) has prominent increased sorption ability for boron within pH range 6.5–7.5. The sorption trend was found to follow Langmuir isotherm model as compared to the Freundlich isotherm.

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Acknowledgments We would like to thank The Scientific and Technological Research Council of Turkey (2216 Research Fellowship Programme For Foreign Citizens) and the Research Foundation of Selcuk University (BAP) for financial support of this work.

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